Originally Posted by
corporate-wage-slave
The trickiest bit is once landside - he needs to go rightwards, through some corridors to the check-in area, then up one level to the Post Office, which is nearly opposite T3 Fast Track. T3 arrivals is actually in a separate building to the rest of T3 and the link is via those corridors.
From the T3 customs exit into the landside arrivals area look far right to the Caffè Nero and go between Caffè Nero and the lifts, through the passage way there to T3 departures. Do not go up or down any stair or escalator until you see some airline checkin counters, indicating you are in the departures building. Then go upstairs (any stair will do) and find the Fast Track entrance. Stand there and look around and the Post Office is visible, almost behind you as you face the Fast Track entrance (as c-w-s says).